mid-14c. (as empyre), probably via Medieval Latin empyreus, from Greek empyros "fiery," from assimilated form of en (see en- (2)) + pyr "fire," from PIE root *paəwr- "fire" (see fire (n.)). As an adjective in English from early 15c. The etymological sense is "formed of pure fire or light." In ancient Greek cosmology, the highest heaven, the sphere of pure fire; later baptized with a Christian sense of "abode of God and the angels."